Since there is a trend of reducing dimensions and weight in automobile valve springs, in order to reduce the diameter of a spring wire while increasing design stress, a necessary degree of strength of the spring wire has been increasing. In this regard, in springs, further improvement of fatigue strength is required for having sufficient fatigue resistance even when high stress is applied to the springs. As one of the means for satisfying this requirement, high compressive residual stress may be provided from a surface to deep inside of a surface layer of a spring wire. Conventionally, in springs, the compressive residual stress is generally provided to a surface layer of the spring wire by shot peening. However, since the amount of plastic strain at the surface layer is decreased according to increase of the hardness of the spring wire in recent years, a thick compressive residual stress layer is difficult to obtain.
By increasing the compressive residual stress at the outermost surface layer by conventional shot peening, breakage originating from the surface at an early time may be prevented. On the other hand, according to the increase in design stress in recent years, combined stress of applied stress and residual stress (net stress applied to an inside of a spring wire) reaches a maximum at around a depth of 200 to 600 μm from the surface. This depth from the surface in a radial direction depends on the diameter of the spring wire, the degree of the applied stress, and the like. If inclusions with sizes of approximately 20 μm exist within this area, stress concentrates on the inclusions. The concentrated stress may exceed the fatigue strength of the spring wire and make the inclusions starting points of breakage. Accordingly, the following techniques were disclosed in order to solve these problems.
A spring with high durability is disclosed in Japanese Unexamined Patent Application Laid-open No. 2009-52144. This spring is subjected to shot peening after gas nitriding, whereby it has a nitrided layer that has a surface portion with compressive residual stress of not less than 1200 MPa. The compressive residual stress is provided from the surface to not less than 250 μm depth. As disclosed in the Examples, the compressive residual stress is provided from the surface to 290 μm depth at most in this spring. Therefore, it is difficult to prevent breakage originating from an area that is deeper than 290 μm. Moreover, since the nitrided layer has little ductility and is brittle, the nitrided layer may facilitate formation of fatigue cracks and cause a decrease in fatigue strength.
A spring with superior fatigue strength is disclosed in Japanese Patent No. 3028438. In this spring, compressive residual stress of 90±10 kgf/mm2 is provided from the surface layer to 150 μm depth. According to the fatigue test disclosed in Japanese Patent No. 3028438, the condition of shear stress was τ=65±50 kgf/mm2. The shear stress is small compared with practical stress conditions (for example, τ=78±73 kgf/mm2) for lightweight and high strength valve springs of recent years.
Another spring with superior fatigue strength is disclosed in Japanese Unexamined Patent Application Laid-open No. 2005-139508. This spring is subjected to shot peening after a nitriding treatment, and it is provided with surface compressive residual stress of not less than 1600 MPa. It is insufficient to greatly improve the fatigue strength only by specifying the degree of the surface compressive residual stress. Preventing internal fractures due to inclusions is rather important, but descriptions relating to compressive residual stress inside the spring are not disclosed.
A spring steel with superior fatigue characteristics is disclosed in Japanese Unexamined Patent Application Laid-open No. 6-158226. The spring steel includes oxide inclusions composed of, by weight %, 30 to 60% of SiO2, 10 to 30% of Al2O3, 10 to 30% of CaO, and 3 to 15% of MgO, and the oxide inclusions have circle-equivalent diameters of not more than 15 μm. However, it is difficult to precisely control the compositions and the grain sizes of the oxide inclusions to be in the above range. In this regard, it is necessary to inspect whether the amounts of the oxide inclusions in produced spring steels are in the above range. In spring steels other than spring steels that are inspected, even if they are of the same lot, the amounts of the oxide inclusions may be out of the above range. In this case, a spring made of the spring steel has a potential of break early due to the oxide inclusions.
Another spring is disclosed in Japanese Unexamined Patent Application Laid-open No. 2003-170353. This spring is subjected to shot peening using amorphous particles as a projection material after a nitriding treatment, and it is provided with maximum compressive residual stress of not less than 1600 MPa. According to the Example disclosed in Japanese Unexamined Patent Application Laid-open No. 2003-170353, the maximum compressive residual stress at the surface of the spring was approximately 2500 MPa. In this case, descriptions relating to a compressive residual stress distribution in depth direction are not disclosed. Estimating from the accompanying FIGURE in the Example, the compressive residual stress was provided from the surface to approximately 250 μm depth. Therefore, it is difficult to prevent internal fractures originating from an area that is deeper than 250 μm.
A carbonitrided quenched material and a production method therefor are disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-46088. The carbonitrided quenched material has a surface layer without nitrogen compounds and has a nitrogen diffused layer from the surface to a predetermined depth where nitrogen is solid solved. In addition, the carbonitrided quenched material is subjected to a quenching treatment. According to this technique, brittle nitrogen compounds that can become starting points of breakage are not formed after nitrogen is absorbed, and the surface layer has high hardness, whereby the fatigue strength may be improved. However, in the invention disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-46088, compressive residual stress is not described, and a high hardness layer at the surface had a thickness of approximately 60 μm at most. Therefore, the fatigue strength cannot be greatly improved only by the technique disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-46088. In addition, according to the production conditions disclosed in Japanese Unexamined Patent Application Laid-open No. 2007-46088, the carbonitriding temperature was low. As a result, the concentration of nitrogen at the surface was low, and a concentrated layer was thin.